The instant invention relates to a method and apparatus for recovering oil, such as contained in a high viscosity crude oil residue, from an oil spill on the surface of a body of water, wherein virtually all of the water initially recovered with the oil is separated therefrom and returned to the body of water.
It is widely recognized that oil spills represent a major threat to the ecological systems associated with many bodies of water, including bays, rivers, coastal areas and areas of open water. For example, it is well documented that an oil spill such as the one which occurred in Prince William Sound off the coast of Alaska in March of 1989 can cause immeasurable damage to coastal areas as well as immeasurable damage to many forms of marine wildlife. Nevertheless, because of the critical importance of petroleum products to virtually all industrialized economies, it is also widely recognized that it is simply not possible to eliminate the practice of transporting crude oils and refined petroleum products on many navigable waterways. Further, it is recognized that while precautionary measures can be taken to reduce the frequency of oil spills on navigable waterways it is virtually impossible to eliminate all oil spills.
Although virtually all oil spills are potentially damaging to the environment, it has generally been found that the most serious oil spills are those in which crude oils or heavy residual oils are spilled onto the surfaces of bodies of water rather than spills involving lighter refined products, such as gasoline, kerosene, or No. 2 fuel. This is because lighter refined products tend to have somewhat higher solubilities in water and they also tend to dissipate relatively quickly through evaporation. Light refined products have also generally been found to be less damaging to shore areas and wildlife than other types of oil. On the other hand, oil spills involving crude oils or heavy residual oils, such as No. 6 fuel, tend to be extremely damaging to coastal areas and marine wildlife. Further, the highly viscous heavy residual fractions of oils of this type, particularly crude oils, tend to rapidly concentrate as the lighter fractions thereof dissipate, making their impact on the environment even more severe.
It has been found that one of the main reasons that damage to the environment from oil spills is so extremely high is the general ineffectiveness of the previously known processes and apparatus for rapidly recovering oils from the surfaces of bodies of water. In this connection, most of the previously known apparatus for recovering oil from the surfaces of bodies of water have comprised skimmer boats which are operative in combination with floating booms for skimming the surfaces of bodies of water to recover oil therefrom. However, because the previously known skimmer boats have often picked up as much as 100 gallons of water for every one gallon of oil recovered, they have been found to be impractical and incapable of rapidly recovering large quantities of oil from the surfaces of bodies of water. Specifically, this is because the previously available skimmer boats have generally been incapable of separating oil from water recovered therewith and as a result, they have required extremely large on board storage capacities to recover relatively small quantities of oil. Further, while skimmer boats which are capable of recovering oil without excessive quantities of water utilizing rotating discs have also been previously available, these boats have been incapable of effectively recovering lighter oils and they have generally been ineffective in other than perfectly calm sea conditions. Hence the previously available skimmer boats have generally been incapable of rapidly recovering large quantities of oil from the surfaces of bodies of water. Still further, while various types of absorbent materials have also been heretofore available for use in recovering oils from the surfaces of bodies of water, it has been found that such materials have generally only been capable of recovering small quantities of oil and hence they have been found to be impractical for use in connection with relatively large oil spills such.
While oil-water coalescers have often been found to be effective for separating oil from water in various on-shore processing operations in oil refineries, petrochemical plants, and the like, the use of such coalescers has generally been limited to processing oil-water streams containing relatively light oils having viscosities of less than approximately 80 centipoise. This is because the rate at which separation between the oil and water phases of an oil-water stream can be effected in an oil-water coalescer is directly related to the viscosity of the oil in the oil-water stream through Stoke's Law. In this regard, as is well known to those skilled in the art relating to oil-water coalescers, the rate at which separation can be effected is directly related to the terminal velocities of the immiscible water droplets in the oil phase. The terminal velocities of the water droplets are determined by Stoke's Law in accordance with the following: ##EQU1## Through experimentation it has been determined that in most cases Stoke's Law effectively limits the practical use of oil-water coalescers to processing oil-water streams having viscosities of less than approximately 80 centipoise. As a result, heretofore it has generally not been considered to be possible to process oil-water mixtures containing oils having viscosities of greater than approximately 80 centipoise through oil-water coalescers. Since the oils recovered from most oil spills involving crude oils or residual oils have viscosities well in excess of 80 centipoise, it has generally not been considered to be possible to utilize oil-water coalescers to process the oils recovered from most crude oil spills. Further, since the heretofore available oil-water coalescer apparatus have not been capable of processing the oils recovered from most crude oil spills, the use of oil-water coalescers in any type of oil spill recovering apparatus has previously not been considered to be practical.
The instant invention represents a significant advancement in the technology relating to oil spill recovery processes and apparatus by providing an effective method and apparatus for rapidly recovering a relatively large volume of oil from an oil spill on the surface of a body of water. More specifically, the instant invention provides a method and apparatus for recovering oil from the surface of a body of water by skimming the surface of the body of water to recover a feed oil-water mixture therefrom, separating the oil in the feed oil-water mixture from the water therein and then returning the separated water to the body of water so that the oil can be practically stored on board a recovery boat, such as a skimmer boat, a barge or any other floating container. Still more specifically, the instant invention provides a method and apparatus which are operative for processing a feed oil-water mixture through an oil-water coalescer to separate the oil in the feed oil-water mixture from the water therein regardless of the viscosity of the oil. Accordingly, when the method and apparatus of the instant invention are utilized on board a navigable boat, such as a skimmer boat, the oil recovered from an oil-water mixture can be immediately separated from the water recovered therewith so that the oil can be stored on the boat without also storing an excessive quantity of water. Hence, the method and apparatus of the instant invention can effectively increase the storage capacity of a skimmer boat by a factor of up to 50 times or more making it possible and practical to rapidly recover a relatively large quantity of oil from the surface of a body of water utilizing a skimmer boat or the like. The method of the instant invention in its broadest form is operative for recovering an oil having a viscosity of greater then approximately 80 centipoise from the surface of a body of water by continuously withdrawing a feed oil-water mixture from the surface of the body of water, continuously adjusting the viscosity of the oil in the feed oil-water mixture to a level below approximately 80 centipoise to form an adjusted oil-water mixture and continuously passing the adjusted oil-water mixture through an oil-water coalescer. However, in the preferred form of the method of the instant invention the viscosity of the oil in the feed oil-water mixture is adjusted by adding a petroleum distillate solvent thereto, such as kerosene or No. 2 fuel oil, in order to reduce the viscosity of the oil in the feed oil-water mixture to a level below approximately 80 centipoise. The method is preferably carried out on board a boat, such as a skimmer boat, which is navigable on a body of water having an oil spill thereon and the water from the oil-water coalescer is preferably returned directly to the body of water, whereas the oil is preferably stored on board the boat. In the preferred form of the method, solvent is added to the feed oil-water mixture at a rate which is substantially greater than that required to reduce the viscosity of the oil in the feed oil-water mixture to a level below approximately 80 centipoise and the oil-solvent mixture from the coalescer is stored in a recycled vessel and then added to the feed oil-water mixture to reduce the viscosity of the oil in the feed oil-water mixture to a level below approximately 80 centipoise. A quantity of oil-solvent mixture equal to approximately 30% of the volume of the feed oil-water mixture during a one (1) minute interval is preferably accumulated in the recycle vessel before the oil-solvent mixture is used to reduce the viscosity of the oil in the oil in the feed oil-water mixture in order to provide a sufficient quantity of oil-solvent mixture and a sufficient residence time of the oil-solvent mixture in the recycle vessel to make it practical to control the viscosity of the feed oil-water mixture with the oil-solvent mixture. In any event, the oil-solvent stream from the recycle vessel is continuously recycled to the feed oil-water mixture for a period of time which is less than the period of time required to raise the viscosity of the oil-solvent mixture in the recycle vessel to a level above approximately 80 centipoise.
The apparatus of the instant invention which is operative for carrying out the method comprises means for withdrawing a feed oil-water mixture from the surface of a body of water, means for adjusting the viscosity of the oil in the feed oil-water mixture to a level of below approximately 80 centipoise to form an adjusted oil-water mixture and an oil-water coalescer for separating the oil in the adjusted oil-water mixture from the water therein. The means for adjusting the viscosity of the oil in the feed oil-water mixture preferably comprises means for adding a hydrophobic hydrocarbon solvent to the feed oil-water mixture, the solvent having a viscosity of less than approximately 80 centipoise and being added at a rate sufficient to reduce the viscosity of the oil in the feed oil-water mixture to a level below approximately 80 centipoise. The means for adding solvent is preferably operative for adding solvent at a rate which is in excess of the rate required to reduce the viscosity of the oil in the feed oil-water mixture to below approximately 80 centipoise and the apparatus preferably further comprises a recycle tank for receiving the oil-solvent mixture from the coalescer and means for recycling the oil-solvent mixture from the recycle tank into the feed oil-water mixture in order to reduce the viscosity of the feed oil-water mixture to a level below approximately 80 centipoise. The apparatus is preferably mounted on a boat such as a skimmer boat, a barge or any other floating container which is navigable on the surface of a body of water and the coalescer preferably includes an inlet first stage, an oil second stage and a water second stage, wherein the inlet first stage is operative for producing a two phase oil-water intermediate stream, the oil second stage is operative for receiving and further purifying the oil phase of the intermediate stream and the water second stage is operative for receiving and further purifying the water phase of the intermediate stream.
Apparatus and methods representing the closest prior art to the subject invention of which the applicant is aware are disclosed in the U.S. Pat. Nos. to Kondoh et al 3,536,616; Hirs No. 3,925,202; Chambers No. 4,168,229; and Duverne No. 4,526,514, however, while the apparatus and methods disclosed in these references are generally related to oil-water recovery and separation apparatus they fail to suggest the concept of adjusting the viscosity of the oil in an oil-water mixture recovered from an oil spill and then passing the adjusted oil-water mixture through an oil-water coalescer and hence, they are believed to be of only general interest with respect to the subject invention.
It has been found that the method and apparatus of the instant invention can be effectively utilized on board a boat, such as a skimmer boat, for separating the oil recovered from an oil spill from the water recovered with the oil so that the oil can be more effectively stored on board the boat. In this connection, it has been found that by reducing the viscosity of the oil in a feed oil-water mixture from an oil spill, the feed oil-water mixture can be effectively processed in an oil-water coalescer regardless of the original viscosity of the oil from the oil spill. In the preferred method and apparatus the viscosity of the oil in the feed oil-water mixture is reduced by adding a solvent to the feed oil-water mixture. This actually has the effect of an oil spill. However, despite the fact that the addition of a solvent increases the volume of the oil and solvent mixture recovered from the coalescer, the volume of the oil and solvent mixture is normally substantially less than the volume of the feed oil-water mixture drawn from an oil spill. Hence, despite the fact that the volume of the solvent has the effect of reducing the storage capacity for storing oil from an oil spill on board a skimmer boat the net effect of the solvent is to substantially increase the overall storage capacity of the skimmer boat by making it unnecessary to store water drawn from a crude oil spill along with the oil recovered therefrom.
As a result of the above, it is a primary object of the instant invention to provide an effective method for recovering oil from an oil spill on the surface of a body of water.
Another object of the instant invention is to provide an effective method of recovering oil from a surface of a body of water wherein water recovered with the oil is returned to the body of water.
An even further object of the instant invention is to provide an effective method and apparatus for separating a relatively high viscosity oil in an oil-water mixture recovered from the surface of a body of water from the water in the oil-water mixture so that the oil can be effectively stored on an oil recovery boat navigating on the surface of the body of the water.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawing.